Modern telephone switching systems have achieved flexibility through the use of program control wherein a processor system, under the control of a program stored in memory, controls the operations of the system. In order to be able to serve a very broad range of office sizes, many modern systems use the technique of distributed control in which the system use the technique of distributed control in which the system is broken down into a number of modules and each module is controlled by its own processor. In one such system using the technique of parallel distributed control, the 5ESS.TM. switch manufactured by AT&T Technologies, Inc., described U.S. Pat. No. 4,322,843 by H. J. Beuscher et al., each module processes all signals, associated with source and destination ports connected to customers or to trunks to other switches, of that module and handles all intramodule switching between ports connected to that module. In addition, this system also uses a central process or for handling a large class of call processing functions associated with intermodule calls or requiring the use of a data base, such as the relationship between directory numbers and ports, that is used by all modules.
In this system, the processors of the various modules communicate with each other via data messages transmitted over the switching network of the system to a message switch which then retransmits each message to the module for which that message was destined. Such an arrangement introduces considerable delay in the transmission of each message. The total delay which can be allowed in processing and responding to incoming signals, such as a customer answer, is limited in commercial telephone systems by telephone requirements according to international standards. Therefore,the number of such data messages which may be exchanged among modules in order to respond to customer requests is limited. This limitation helps to determine the assignment of functions to module processors and the central processor.
Because the central processor is used for performing many of the basic data processing functions required to set up each telephone call, it is necessary that a highly reliable central processor be available in each switch. Further, these substantial data processing requirements lead to the requirement of a processor having very high processing capability in order to handle to very large office. It is an objective of distributed control telephone switching systems to be able to grow readily, by the addition of individual switching modules, one module for each growth step, from a small size of one switching module serving perhaps 5,000 customers to sizes having many modules and serving in excess of 100,000 customers.
In summary, a problem exists in the prior art of parallel distributed processing telecommunication systems in that a reliable high-capacity central processor is required with substantial costly and complex common software resident in that processor to support a systems architecture for small switching offices while maintaining a capability of growing to a large office.